Microwave sensor

ABSTRACT

A microwave sensor is described, with a self-mixing oscillator ( 1 ), with a transmitting and receiving antenna ( 2 ), with an impedance ( 3 ) which is connected between the current or voltage supply ( 4 ) and the oscillator ( 2 ), and with an evaluation circuit ( 5 ), the self-mixing oscillator ( 2 ) producing both the transmitted signal and also mixing the transmitted signal with the received signal and the low-frequency mixed (Doppler signal) being tapped on the impedance ( 3 ) and supplied to the evaluation circuit ( 5 ).  
     The microwave sensor has only lower power consumption, and the microwave sensor can also be economically produced in that the self-mixing oscillator ( 2 ) is made as a push-pull oscillator with two transistors ( 6, 7 ).

[0001] The invention relates to a microwave sensor with a current orvoltage supply, with a self-mixing oscillator, with an impedance whichis connected between the current or voltage supply and the oscillator,with a transmitting and receiving antenna and with an evaluationcircuit, the self-mixing oscillator producing both the transmittedsignal and also mixing the transmitted signal with the received signaland the low-frequency mixed product being tapped on the impedance andsupplied to the evaluation circuit.

[0002] In the first type of microwave sensors of the above describedtype, the movement of an article in an area to be monitored isascertained by the so-called Doppler effect being evaluated. Atransmitted signal radiated by the transmitter with a frequency f₁ isreflected by an object moving in the area to be monitored. Part of thereflected signal is incident as the received signal with a frequency f₂on the receiver. In a suitable mixer the received signal is mixed withthe transmitted signal and then the portion of the mixed product withthe Doppler frequency f_(D) is evaluated. The following equations applyto the Doppler frequency f_(D)

f _(D) =f ₁ −f ₂ and

f _(D)=[(2×f ₁)/c ₀ ]×v _(r)

[0003] c₀ being the velocity of light and v_(r) being the radialvelocity of the moving object.

[0004] Thus either the radial velocity v_(r) of the moving object can bemeasured from the measured Doppler frequency f_(D) or, if the microwavesensor is used only for monitoring a space or a certain area, the entryof an object into the space or area to be monitored can be ascertained.One such microwave sensor which evaluates the Doppler effect and whichis often also called a Doppler sensor can thus be used as a motiondetector for the most varied applications.

[0005] In a second type of microwave sensors of the above described typefor detection of a moving or stationary object in an area which is to bemonitored, a frequency-modulated transmitted signal with a frequencyf₁(t) is emitted by a transmitter. The modulation signal is produced indoing so by a suitable modulation generator, and the frequency f₁(t) ofthe transmitted signal can change linearly, sinusoidally or according toanother time function. This radar process is called FMCW radar(FMCW=frequency modulated continuous wave).

[0006] Here the transmitted signal with a frequency f₁(t₀) is reflectedby an object which is located in the area to be monitored. Part of thereflected signal after a time interval At is incident as the receivedsignal with frequency f₁(t₀) on the receiver. At this time thetransmitted signal already has the frequency f₁(t₀+Δt). The receivedsignal in its frequency thus runs behind that of the transmitted signal.In a suitable mixer the received signal is mixed with the transmittedsignal and then the portion of the mixed product with frequency f_(IF)is evaluated.

[0007] The microwave sensor under consideration can be used both as aDoppler sensor and also as a FMCW sensor. In doing so the frequency ofthe transmitter depending on the application can be between 60 MHz and60 GHz and thus also somewhat below the actual microwave range whichnormally extends from 300 MHz to 300 GHz. Strictly speaking, the sensorunder consideration is a radio or microwave sensor.

[0008] It was stated at the beginning that the microwave sensor has aself-mixing oscillator and a transmitting and receiving antenna. Aself-mixing oscillator, which can also be called a self-oscillatingmixer, is a component which is used both as an oscillator and also as amixer. On the one hand, therefore either the self-mixing oscillatorproduces a transmitted signal, on the other in the self-mixingoscillator which then operates as a self-oscillating mixer, the receivedsignal is mixed with the transmitted signal. Within the framework ofthis application a transmitting and receiving antenna is defined as acomponent which is used at the same time as a transmitting antenna andas a receiving antenna.

[0009] One such microwave sensor in which instead of the fourcomponents—oscillator, mixer, transmitting antenna and receivingantenna—only the above described two components—a self-mixing oscillatorand transmitting and receiving antenna are used, is disclosed by DE 3209 093 A1 and DE 41 27 892 A1. In the known microwave sensors theself-mixing oscillator is formed by a feedback field effect transistorwith a resistor for tapping the Doppler signal in its source-draincircuit. With the known microwave sensor it has already been possible tomake available a device for space monitoring by means of Doppler radarwhich requires only relatively few components and therefore can beproduced both economically and also has only little space requirementand low weight.

[0010] Especially when one such microwave sensor is made as a 2-wiredevice or is to be battery-operated is there however the problem thatthe known microwave sensors have an overly great power consumption or atlow power consumption have an overly low transmitted power. One suchmicrowave sensor made as a 2-wire device can also be called a microwaveproximity switch.

[0011] Therefore the object of this invention is to improve theinitially described microwave sensor such that it has lower powerconsumption and can be produced as economically as possible.

[0012] This object is first achieved in the initially describedmicrowave sensor essentially in that the self-mixing oscillator is madeas a push-pull oscillator with two transistors.

[0013] The use of a push-pull oscillator has the advantage that in thisway higher power and better efficiency can be achieved so that one suchpush-pull oscillator builds-up well even at a relatively low voltage orrelatively low current. In particular a symmetrically built push-pulloscillator compared to known field effect transistors used asself-mixing oscillators is much more oscillation-friendly, additionallyfew harmonics also occurring.

[0014] The transmitting and receiving antenna is advantageously formedby a strip line which determines the frequency of the oscillator. Byusing a strip line as the transmitting and receiving antenna which is acomponent of the push-pull oscillator it is possible to save anothercomponent, since a separate transmitting and receiving antenna is nolonger necessary. In addition, however it is also possible to use adipole antenna as the transmitting and receiving antenna.

[0015] According to one preferred embodiment of the microwave sensor asclaimed in the invention, to adjust or stabilize the working point ofthe microwave sensor, voltage countercoupling with at least one resistorand one lowpass is accomplished.

[0016] In the Doppler sensor known from DE 41 27 892, A1, to stabilizethe working point of the transistor, current countercoupling isimplemented, for which between the drain terminal and the gate terminalof the field effect transistor and ground one impedance at a time isconnected. This current countercoupling for adjusting the working pointleads on the one hand to an undesirable cross current; this increasesthe energy demand of the microwave sensor which is required overall. Onthe other hand, when the working point is adjusted by means of currentcountercoupling there is the danger that the useful signal, i.e. theDoppler signal, is “regulated out” or at least attenuated bycountercoupling.

[0017] Because the working point is adjusted by voltage countercoupling,first of all an unwanted cross current is avoided. In addition, thelowpass which is provided ensures that only possible temperature driftis corrected, but the Doppler signal is not attenuated. To do this, thelowpass is adjusted such that working point control is slower than thelowest expected Doppler frequency f_(D) or the lowest intermediatefrequency f_(IF). In a transmitted signal with a frequency f₁ of a fewgigahertz, for example 2.5 GHz, and tuning of the microwave sensor tomovements executed by individuals, for example a hand approaching adoorknob, the Doppler frequency f_(D) is for example between 10 and 50Hz so that the cutoff frequency of the lowpass must accordingly bechosen to be smaller than 10 Hz. Preferably the cutoff frequency of thelowpass is roughly ⅓ of the lowest expected Doppler frequency f_(D) orthe lowest intermediate frequency f_(IF). When the working point isadjusted or stabilized in this way, only very few components are needed.In particular, the resistor which is used for voltage countercouplingcan be at the same time also a component of the lowpass. Then only oneresistor and one, preferably two capacitors, are needed for workingpoint control.

[0018] In one alternative embodiment of the microwave sensor as claimedin the invention, the working point is stabilized or adjusted usingcurrent control with at least one lowpass. The lowpass ensures in turnthat only possible temperature drift is corrected, but the Dopplersignal or the intermediate signal is not attenuated. The current iscontrolled via changing the base current of the oscillator transistorsof the push-pull oscillator. Here the working point control can beconnected directly to the bases of the oscillator transistors or to themiddle terminal of the strip line.

[0019] According to another embodiment of the invention, the workingpoint is controlled using a comparator and a lowpass or an integrator.The comparator is for example a correspondingly wired operationalamplifier. Control which has been structured in this way takes placewith a frequency which is smaller than the smallest expected Dopplerfrequency f_(D). Since only possible temperature drift of thetransistors is to be compensated by working point adjustment orcontrol—the base emitter voltage of a transistor belonging to a givencollector current decreases by roughly 2 mV per degree of temperatureincrease—working point control with a frequency of only a few hertz issufficient.

[0020] It was stated at the beginning that the microwave sensor has animpedance which is connected between the current or voltage supply andthe self-mixing oscillator, the Doppler signal being tapped on theimpedance and supplied to the evaluation circuit. If the microwavesensor as claimed in the invention is to have only minimum powerconsumption and is to be built with costs as low as possible and thusalso with as few components as possible, the impedance can be easilymade as an ohmic resistance. One such microwave sensor is especiallysuited for battery operation since the microwave sensor then only has apower consumption of less than 0.5 mA. Based on the veryoscillation-friendly push-pull oscillator, even a lower current foroperation of the self-mixing oscillator is sufficient. One suchmicrowave sensor can be integrated in the door handle of a motor vehiclewhere the microwave sensor establishes the approach of a hand to thedoor handle and upon response activates a transponder for interrogationof the access authorization. Likewise, one such microwave sensor can beused as a proximity sensor in sanitation systems—toilet flushing or handdriers—or in pedestrian traffic lights. With one such microwave sensorwhich is optimized to minimum power consumption and which in additionalso has only very small dimensions, only a very small range of a fewcentimeters—up to roughly 10 to 20 cm—can be implemented so that themicrowave sensor can also monitor only a correspondingly small area ofspace which is however sufficient for the aforementioned applications.

[0021] If a greater range is to be achieved with the microwave sensor,but the power consumption is to be kept as small as possible,advantageously an impedance is used which has a frequency-dependentresistance. The impedance is designed such that it is low-resistance forthe DC voltage or direct current supply of the self-mixing oscillator,but is as high-resistance as possible for the Doppler signal or theintermediate signal. One such impedance can be formed for example by aconstant current source.

[0022] The use of an impedance with the above describedfrequency-dependent resistance has the advantage that on the one hand arelatively large amount of current can be supplied to the transistors ofthe push-pull oscillators due to the low resistance for direct currentpower supply so that a relatively high transmitted power is available.On the other hand, one such impedance due to its high resistance for theDoppler signal has the advantage that high gain is achieved.

[0023] One such impedance can also be implemented by a reactor,especially an electronic reactor, for example a gyrator. A gyrator hasthe advantage that it has very low resistance for direct current supplyfor the LF signal, but is very high resistance for HF signals; thisleads to the desired high gain of the useful signal.

[0024] To increase the frequency stability of the push-pull oscillator,the strip line can be connected to a dielectric resonator. One suchdielectric resonator then due to its high quality assumes frequencyguidance of the push-pull oscillator. The use of a dielectric resonatoris advantageous especially when the microwave sensor is to be used inthe immediate vicinity of other electronic devices and circuits, so thatit must be ensured that these electronic devices and circuits are notadversely affected by the radiated transmitted signal.

[0025] The strip line is made advantageously as a λ/2 (lambda half)microstrip line. The tuning of the length of the microstrip line to thefrequency f₁ of the transmitted signal has the advantage that when thelength of the microstrip line is chosen according to half the wavelengthof the transmitted signal, the voltage zero point of the sinusoidaltransmitted signal is in the geometric middle of the microstrip line, bywhich the connection for working point control can be easilyaccomplished, especially reactors for injection are not necessary. Toimprove the directional characteristic of the strip line it has a backcopper coating, by which the emission characteristic of the strip linecorresponds roughly to an ellipsoid of rotation.

[0026] Basically there are various possibilities for embodying theevaluation circuit to which the Doppler signal is sent. According to onepreferred embodiment of the invention the evaluation circuit has atleast one amplifier, at least one bandpass and at least one comparator.

[0027] The bandpass is set to the expected frequency f_(D) of theDoppler signal in the range from 10 to 40 Hz so that both noise and also50 Hz hum can be suppressed by the bandpass. Due to the relatively lowfrequency f_(D) of the Doppler signal which is to be evaluated, theamplifier which is advantageously made as two-stage operationalamplifier has only a low power consumption. To ensure response of themicrowave sensor as fast as possible, the comparator is made preferablyas a window comparator so that the comparator has two thresholds, both apositive and also a negative threshold. In this way, any motion of anobject within the monitored area immediately produces a signal at theoutput of the comparator, regardless of the direction of motion. The useof an evaluation circuit with a window comparator moreover has theadvantage that the window comparator can be wired such that it workswithout a closed-circuit current. Only when the microwave sensor isactuated, i.e. when an object is moving in the monitored area of themicrowave sensor, does a load current flow through the windowcomparator.

[0028] Using a voltage regulator which is connected between the currentor voltage supply and the impedance, with a load resistor connected toits output, the use of an otherwise more complex end stage can beomitted. When the microwave sensor is actuated so that the windowcomparator becomes conductive and thus a load current flows via the loadresistor, a voltage dip of the voltage regulator associated with it isimmediately compensated by a corresponding increase of current. Sincethe load resistor is connected to the output of the voltage regulator,the current which flows when the microwave sensor is actuated is exactlydefined.

[0029] In particular there is a host of possibilities for embodying anddeveloping the microwave or radio wave sensor as claimed in theinvention. In this regard reference is made both to the claims which aresubordinate to claim 1 and also to the description of preferredembodiments in conjunction with the drawings.

[0030]FIG. 1 shows an elementary diagram of one embodiment of themicrowave sensor as claimed in the invention,

[0031]FIG. 2a shows an elementary diagram of part of a microwave sensoras shown in FIG. 1,

[0032]FIG. 2b shows an elementary diagram of part of one alternativeembodiment of the microwave sensor as claimed in the invention,

[0033]FIG. 3 shows a block diagram of part of the microwave sensor asshown in FIG. 2b,

[0034]FIG. 4 shows a block diagram of a microwave module of themicrowave sensor and

[0035]FIG. 5 shows a structure of the microwave module as shown in FIG.4.

[0036] According to the elementary diagram shown in FIG. 1, themicrowave sensor as claimed in the invention has a current or voltagesupply 1, a self-mixing oscillator 2, an impedance 3 which is connectedbetween the current or power supply 1 and the self-mixing oscillator 2,a transmitting and receiving antenna 4 and an evaluation circuit 5. Theself-mixing oscillator 2, which produces both the transmitted signal andalso mixes the transmitted signal with the received signal, formstogether with the transmitting and receiving antenna 4 the highfrequency microwave module 6, of which FIG. 4 shows a block diagram andFIG. 5 shows a layout diagram.

[0037]FIGS. 4 and 5 show that the self-mixing oscillator 2 is made as asymmetrical push-pull oscillator with two bipolar transistors 7, 8 andthe transmitting and receiving antenna 4 is formed by a strip line 9.The strip line 9, which is made as a λ/2 microstrip line, is connectedwith its two ends each to the base terminal 15 and 16 of the two bipolartransistors 7, 8.

[0038] In the microwave module 6 which is shown in FIGS. 4 and 5 andwhich is made and structured such that the microwave sensor has onlyminimum power consumption, and to set or stabilize the working point ofthe transistors 7, 8, voltage countercoupling 10 is accomplished withone resistor 11 and two lowpasses 12. For the two lowpasses 12 of thetwo transistors 7 and 8 the same resistor 11 is used as also is used forvoltage countercoupling as well.

[0039] One alternative embodiment of voltage countercoupling 10 whichconsists of one voltage comparator 13 and one integrator 14 is onlysuggested in FIG. 1. The integrator 14 is used, in the same way as theRC elements which are shown in FIGS. 4 and 5, as a lowpass with a cutofffrequency below the Doppler frequency f_(D). The impedance 3 which islikewise only shown schematically in FIG. 1 can be accomplished by anohmic resistance, a constant current source 3′ or by a gyrator 3″.Preferably in the current- and component-optimized embodiment of themicrowave sensor which has a microwave module 6 as shown in FIGS. 4 and5, only one ohmic resistance is used as the impedance 3. Conversely,when the microwave sensor is to have a somewhat greater range, theimpedance 3 is accomplished by a constant current source 3′ or by agyrator 3″.

[0040] As can be seen in FIG. 2, when the impedance is a constantcurrent source 3′ (FIG. 2a) the tap of the working point control, i.e.the voltage countercoupling 10, takes place above the oscillator 2,while when the impedance is a gyrator 3″(FIG. 2b) the tap of the currentcontrol 10′ takes place below the oscillator 2. The current control 10′consists of an amplifier 13′ and a lowpass 14′.

[0041] The evaluation circuit 5 which is shown only schematically inFIG. 1 consists of an amplifier 17, a bandpass 18 and a windowcomparator 19. The amplifier 17 can be made as a two-stage operationalamplifier, the bandpass 18 being implemented by RC countercoupling ofthe operational amplifier. The circuit of the microwave sensor moreoverhas a voltage regulator 20 with a load resistor 22 connected to itsoutput 21. When the microwave sensor is actuated, i.e. an object movesinto the monitored area, the window comparator 19 becomes conductive, sothat a load current flows via the load resistor 22. The associatedvoltage dip by the voltage regulator 20 is immediately compensated by acorresponding rise of the current. The voltage regulator 20 thusprovides for a constant operating voltage of the microwave sensor, forthe 2-wire microwave sensor shown in FIG. 1 the output signal being onthe current or voltage supply I in the form of two different currents,closed-circuit current or increased current, with the microwave sensoractuated. Finally, FIG. 1 shows a protective circuit 23 which consistsof a transistor as a preregulator and of diodes as overvoltage andreverse voltage protection.

[0042] As is shown in FIGS. 2, 3, and 4, the oscillator 2 is connectedvia a filtering and matching network 24 to ground in order to achievegain of the useful signal as high as possible. The filtering andmatching network 24 contains especially one inductance and alow-resistance resistor connected in series to it per emitter branch.

[0043] In order to be able to build the microwave module 6 as easily aspossible and with as few components as possible, the collectors 25, 26of the transistors 7, 8 are blocked by fan-shaped triangles 27 asblocking capacitors against the rear copper coating of the microwavemodule 6. The rear copper coating of the microwave module 6 improves theemission characteristics of the strip line 9, for which the free area onthe top 28 of the microwave module 6—especially adjacent to the stripline 9—can be filled with copper. The top surface 28 of the microwavemodule 6 is connected via contact-making points 29 to the rear coppercoating on the bottom. As moreover can be seen from FIG. 5, the highfrequency reactors are implemented by meandering lines 30. Additionalcomponents such as capacitors or high frequency reactors can thus beomitted or can be implemented especially easily by means of thesemeasures.

1. Microwave sensor with a current or voltage supply (1), with aself-mixing oscillator (2), with an impedance (3) which is connectedbetween the current or voltage supply (1) and the oscillator (2), with atransmitting and receiving antenna (4) and with an evaluation circuit(5), the self-mixing oscillator (1) producing both the transmittedsignal and also mixing the transmitted signal with the received signaland the low-frequency mixed product being tapped on the impedance (3)and supplied to the evaluation circuit (5), characterized in that theself-mixing oscillator (2) is made as a push-pull oscillator with twotransistors (7, 8).
 2. Microwave sensor as claimed in claim 1, whereinthe transmitting and receiving antenna (4) is formed by a strip line (9)which determines the frequency of the push-pull oscillator.
 3. Microwavesensor as claimed in claim 1 or 2, wherein to adjust or stabilize theworking point, voltage countercoupling (10) is implemented with at leastone resistor (11) and with one lowpass (12).
 4. Microwave sensor asclaimed in claim 1 or 2, wherein to adjust or stabilize the workingpoint, current control (10′) is implemented with at least one resistor(11) and one lowpass (14′).
 5. Microwave sensor as claimed in claim 1 or2, wherein a comparator (13) and a lowpass or an integrator (14) aredesigned for stabilization of the working point.
 6. Microwave sensor asclaimed in one of claims 1 to 5, wherein the impedance (3) is made as anohmic resistance.
 7. Microwave sensor as claimed in one of claims 1 to6, wherein the impedance (3) has a frequency-dependent resistance value.8. Microwave sensor as claimed in claim 7, wherein the impedance (3) ismade as a constant current source (3′).
 9. Microwave sensor as claimedin claim 7, wherein the impedance (3) is made as an electronic reactor,especially as a gyrator (3″).
 10. Microwave sensor as claimed in one ofclaims 2 to 9, wherein bipolar transistors are used as the transistors(7, 8), and the strip line (9) is connected to the base terminal (15,16) of the two bipolar transistors (7, 8).
 11. Microwave sensor asclaimed in one of claims 2 to 10, wherein the strip line (9) isconnected to the dielectric resonator.
 12. Microwave sensor as claimedin one of claims 2 to 1 1, wherein the strip line (9) is made as a λ/2microstrip line and has rear copper coating.
 13. Microwave sensor asclaimed in one of claims 1 to 12, wherein the evaluation circuit (5) hasat least one amplifier (17), at least one bandpass (18) and at least onecomparator, especially a window comparator (19).
 14. Microwave sensor asclaimed in claim 13, wherein the amplifier (17) is made as two-stageoperational amplifier with RC countercoupling.
 15. Microwave sensor asclaimed in claim 13 or 14, wherein the window comparator (19) isde-energized in the idle state.
 16. Microwave sensor as claimed in oneof claims 1 to 15, wherein there is a voltage regulator (20), thevoltage regulator (20) is connected between the current or voltagesupply (4) and the impedance (3) and a load resistor (22) is connectedto the output (21) of the voltage regulator (20).